The National Football League recently slapped Nike for unsportsmanlike conduct for a threatening letter the company sent to hundreds of football players it has under contract warning them against wearing Reebok hats on the sidelines during games.

Reebok has supplied the NFL with game uniforms – including team jerseys and hats that bear the company’s logo – since 2001, when it supplanted Nike as the league’s exclusive apparel supplier.

Nike has a separate deal with the NFL to provide players with footwear.

The flap is the latest example of what experts call ambush marketing – when one company tries to downplay the effectiveness of a rival’s sponsorship deal. Nike spokesjocks Charles Barkley and Michael Jordan had to cover Reebok logos on their Team USA warm up suits with American flags during the 1992 Olympics.

Still, the NFL’s willingness to stand up to Nike – which it claims is using ambush marketing to get the benefits of an apparel deal that it does not pay for – is unusual, analysts said.

“Nike has the upper hand in every one of its business relationships, except with the NFL where it’s a Mexican standoff,” said John Horan, publisher of Sporting Goods Intelligence.

Nike warned that any players wearing Reebok caps during games or press conferences would be in violation of their Nike football contracts, people who have seen the letter said.

Nike also urged players to wear garments bearing Nike logos under their jerseys – in violation of NFL rules that require players to wear only team issued merchandise during games and post-game interviews, these people said.

Gene Upshaw, head of the NFL player’s association, fired back with his own letter, in which he stated that it was inappropriate and unacceptable for Nike to threaten any player for wearing part of the official uniform, including caps, these people said. Players get a percentage of all Reebok NFL caps sold.

Upshaw also reassured players that the association would back them up in court if Nike tried to make good on its threat.

“Nike has a shoe deal and not an apparel deal with the NFL – and they are trying to have it otherwise,” said Doug Allen, assistant executive director of the association.

A Nike spokesman said the letter was similar to one it sends every year that asks players to honor their contracts.